The mainstream phone manufacturers are looking for hits in the mass market, and looking for large niches to broaden their market. However if you take a "long tail" viewpoint, everyone wants a slightly different phone, and many people have phones that are the least bad one they could find.
How about phones for people with poor eyesight? If you are over 50, can you read the small print on your phone's screen without reading glasses? I have a friend who has a medical condition that causes very poor eyesight, and sets her 17" laptop screen resolution very low so she can read it. She would like a phone that has just the applications she needs, big easy to find icons for them, and big fonts on a large screen for the address list and other applications. Hard to find in the shops, but easy to custom build.